Process and device for the recovery of energy

ABSTRACT

The invention relates to a process and device for the recovery of energy from the waste heat of thermal or chemical processes, wherein at least a portion of said waste heat evaporates a liquid via at least one heat transfer means or heats a vapour or a gas, increasing the pressure thereof, and this pressure is transformed into mechanical energy in an engine ( FIG. 1 ).

The invention relates to a process according to the preambles of claims1 and 2 and in each case to a device for carrying out these processes.

Processes for the recovery of energy from exhaust gases in thelarge-scale commercial section of industrial plants are known from theprior art, wherein essentially stationary processes yield acomparatively constant exhaust gas stream, which usually flows directlyback to the process via a recirculation process. The so-calledcogeneration, wherein the thermal energy arising, e.g., in a steam plantis used directly for heating purposes or as process heat, is, by far,more widely used.

In U.S. Pat. No. 5,896,738, for instance, a system for the generation ofsteam from the exhaust gas of a gas turbine is described, wherein thesuperheated steam mixed with fuel is returned to the turbine. Thissystem makes sense in large stationary plants with optimized efficiency.In a mobile use under variable load conditions, the additional waterconsumption would be unacceptable on the one hand and, on the otherhand, the efficiency gain for the auxiliary energy would be forgone.

U.S. Pat. No. 4,729,225 describes a system wherein the turbo charger isdesigned for such an amount of excess energy that said energy can beused for auxiliary drive purposes. Such a solution has the drawback thatit has a direct impact on the design of the combustion engine and,reciprocally, depends more strongly on the operating condition thereofand hence cannot be used as an independent system for the generation ofauxiliary energy.

Document WO 02/31319 discloses a Rankine-process device for an internalcombustion engine, wherein energy is recovered from the waste heat of aprocess. In the abstract, it is explained that a portion of the wasteheat evaporates a liquid via a heat transfer means, increasing thepressure thereof, and this pressure is transformed into mechanicalenergy in an engine.

In a first stage, water is thereby preheated in a first heat exchangerin the exhaust gas. The preheated water is guided around the cylinderblock to a water jacket. Thereupon, a steam turbine transforms thepressure into mechanical energy.

The documents U.S. Pat. Nos. 5,327,987, 5,609,029, WO 94/28298, U.S.Pat. No. 6,155,212, JP2001-132538 and U.S. Pat. No. 4,470,476 also eachdisclose a device and a process of a similar type.

The exhaust gas recirculation (EGR) is a known process in order to beable to reduce the undesired NOx-emissions in the exhaust gas of(diesel) motor vehicles or other means of transport such as ships etc. Aportion of the exhaust gases is returned to the combustion air or to thefuel/air mixture, respectively, via the engine's suction system. Atemperature decrease and a delay in the combustion and hence a reductionin the discharge of nitrogen oxide by approx. 40% are feasible; as arule, the EGR is also associated with a slightly higher consumption offuel.

As is known, the exhaust gases of the combustion engines of freight andpassenger vehicles reach temperatures of 700 or 450° C., respectively.Those hot exhaust gases must be cooled to temperatures in the order of150 to 200° C. so that it is possible to return those gases, which aremixed with combustion air, to the engine. A temperature decrease in theexhaust gas is feasible via the incorporation of a heat exchanger, and,in a standard design, this is indeed constructed in that way.

In the heat exchanger, for example the coolant which cools also thecombustion engine itself can be located. The coolant then flows in amachine-cooling system-loop: First, it absorbs heat from the engine andsubsequently also from the exhaust gas in order to finally release heatinto the environment via a radiator. However, in this system, very highdemands are made both on the heat exchanger and on the radiator (compactdesign, material resistance against high temperatures, corrosion anddepositions) due to the increased temperatures.

In EP 1 091 113 A, possibilities are shown which avoid or at leastminimize the problems just described. For example, the incorporation ofa second high-temperature exhaust gas cooler leads to the absorption ofa large portion of the heat, resulting in that the actualmachine-cooling system-loop can operate as usual and that norestrictions due to the high temperatures have to be imposed. Thissecond exhaust gas cooler is provided in a cooling loop comprising asecond radiator. An altogether more effective EGR-cooling can beachieved, which, in addition, is not necessarily associated with anincrease in the radiator surface.

In any case, thermal energy must be withdrawn from the hot exhaust gasso that it can be used in an EGR in such a way that a reduction in thedischarge of nitrogen oxide will occur. By means of the known methods,the temperature of the exhaust gas can be brought to the required value,however—and that is clearly the great potential of the invention—theenergy of the exhaust gas is merely discharged without being intendedfor any further use.

Therefore, the invention has the task, namely, first of all, ofobtaining a reduction in the thermal load for the cooling system bycoverting the thermal energy of the exhaust gas in the exhaust gasrecirculation into mechanically usable energy and, secondly, of creatinga use of the system in terms of a further decrease in the emissions ofthe combustion engine.

According to the invention, this task is achieved in that at least aportion of the waste heat of the recycled exhaust gas evaporates aliquid and/or heats a vapour and/or a gas, increasing the pressurethereof, and this pressure is transformed into mechanical energy in anengine. Advantageous variants thereof are illustrated in the dependentclaims 3 to 15.

According to a variant, in a process for the recovery of energy from thewaste heat of a combustion engine, in particular of a mobile combustionengine, and of a fuel cell, at least a portion of the waste heat of theexhaust gas of the combustion engine, in particular of a recycledexhaust gas, and at least a portion of the waste heat of the fuel cellevaporate a liquid and/or heat a vapour and/or a gas, increasing thepressure thereof, and this pressure is transformed into mechanicalenergy in an engine. Advantageous variants are included in the dependentclaims 3 to 16.

Claims 17 to 32 include preferred embodiments of devices for carryingout the claimed processes.

Solutions wherein, under variable practical operating conditions, thewaste heat of a combustion engine comprising an exhaust gasrecirculation and/or of a fuel cell, is transformed into an energydifferent from thermal energy, have not been used so far. Thereby, theuse as auxiliary energy for different consumers present in connectionwith the primary chemical or thermal process must be mentioned asparticularly advantageous. Those consumers may require mechanicalenergy, such as, for example, a compressor for an air-conditioningsystem, or also electrical energy, such as, for example, servo motors inthe control process, or the lighting of a vehicle. The specificadvantage in terms of energy technology consists, for example, in thatthe use of energy produced via the primary chemical or thermal process,respectively, is always subject to the full losses of the process, i.e.,any energy withdrawn productively will always produce further wasteenergy whereas the use of lost energy from the exhaust gas will notcreate any further demand for primary energy. If efficiencies of 10 to35% are regarded as typical for an internal combustion engine whichsometimes even has to be kept in operation specially for the requiredauxiliary energy, the energy recovered by the present invention saves,as a primary energy input, three to ten times as much.

Solutions wherein the auxiliary energy of a vehicle is generated, e.g.,via a separate small diesel engine or, e.g., also via a fuel cell arelikewise known from the literature. Both types of auxiliary energysources (APU=Auxiliary Power Unit) exhibit comparatively large heatlosses.

Therefore, the present invention makes use of the excess energy of wastebeat arising in a vehicle driven by an internal combustion enginecomprising an exhaust gas recirculation, by supplying the same via athermal intermediate circuit, preferably involving superheated steam, toan additional engine, preferably a steam turbine, and by withdrawingmechanical energy either directly at the output of the steam turbine ortransforming the same into electric current via a generator known perse. In the same manner, the waste heat of the auxiliary energy sourcesis used for energy utilization.

Depending on the process and design of the thermal or chemical process,respectively, temperatures of 300° C. to 1000° C. can be used for therecovery of energy. In combustion engines, particularly the exhaust gasis usable and is generally available at 300 to 600° C. Similarly, fuelcells, if designed as high-temperature fuel cells, also have highexhaust gas and coolant temperatures, which can reach up to 1000° C.High-temperature fuel cells are also used because they have a slightlyhigher efficiency and are more tolerant in terms of the supplied fuel.However, as a guide value, it can also be assumed that approx. 50% ofthe supplied energy will go into the exhaust gas or is available fromthe cooling process.

In the thermal intermediate circuit, a medium, optionally pressurized,circulates, which, via heat transfer means, absorbs the thermal energyfrom the exhaust gas and/or the cooling circuit of the thermal orchemical process and subsequently releases the same in the additionalengine. Such a medium can be any liquid suitable for a cooling orheating circuit, or a vapour or a gas. Since a mobile plant occasionallyalso has to be operated at temperatures below 0° C., the medium ischosen such that it does not solidify at ambient temperatures normal forvehicles. A simple and proven example thereof is water mixed withantifreeze. A particularly favourable embodiment provides that thethermal intermediate circuit is connected directly to the coolantcircuit of the internal combustion engine, the same medium is used andthe through-flow between the two circuits can be controlled via, e.g., avalve. In this way, on the one hand, the medium cooled after the enginecan contribute to the cooling of the internal combustion engine, and, onthe other hands the medium preheated by the internal combustion enginecan reach a higher temperature after the heat transfer means from theexhaust gas. This means that, in addition, another portion of thethermal energy flowing into the cooling circuit of the internalcombustion engine is recovered. Reciprocally, the energy recovered fromthe exhaust gas of the auxiliary energy source, for instance of a fuelcell, can also be used for preheating the internal combustion engineprior to the start. This guarantees a reduced exhaust-gas dischargeduring the cold start and, optionally, also a preheating of thepassenger compartment via the conventional beating of the vehicle.

In a further advantageous embodiment, the medium of the thermalintermediate circuit is heated in at least two stages. The waste heat ofthe combustion engine is used for preheating in a first stage, and thewaste heat of a second thermal or chemical process, for example of theauxiliary energy source, heats the medium in a second stage to thehigher final value for the supply to the additional engine. By means ofthis design comprising at least two stages, the efficiency of the devicefor the recovery of energy from the exhaust gas can be increasedsubstantially, since the inlet temperature into the engine is higher.

In the normal case, the heating of the medium is performed via heatexchangers in one of the usual designs. A particularly advantageousembodiment of a heat exchanger consists in that the ratio of surface tovolume is maximized via extremely fine metal structures. In doing so,the gas flow control is chosen such that laminar streams, which reducethe heat transfer, are prevented from occurring. Heat exchangers havethe effect that the temperature of the medium in the intermediatecircuit is always cooler than the waste heat used for the heat transfer.Thus, if the internal combustion engine has an exhaust gas temperatureof, e.g., 300° C. at the location where the exhaust gas can be guidedinto the heat exchanger without negative repercussions on the combustionprocess, the medium in the intermediate circuit can reach only about260-280° C. In one embodiment of the invention it is therefore suggestedthat a heat pump is used as a heat transfer means either instead of orin addition to a heat exchanger. Thereby, the temperature of the mediumand the heat content thereof can be increased clearly beyond those ofthe exhaust gas of the internal combustion engine. This allows, in turn,an improved efficiency of the engine, preferably the steam turbine.

A preferred embodiment of the invention is described below.

According to FIG. 1, after the possibly provided turbo charger, theexhaust gas of a combustion engine 1 is guided through a first heattransfer means 2, prior to proceeding to the further exhaust gasaftertreatment and to the exhaust. In the heat transfer means 2, it thusheats a medium 3, preferably the condensate of a water/antifreezemixture, which thereby forms superheated steam. The medium 3 is passedon to a possible second heat transfer means 4, which is charged on theprimary side, e.g., by the exhaust gas or the coolant of a fuel cell 5,thus producing an additional overheating of the medium 3.

Alternatively or additionally, the heat from an exhaust gasrecirculation 13 of the combustion engine 1 can also be supplied to aheat transfer means, preferably to the second one 4—optionally also toanother one.

In the steam cycle, an energy store 6 makes sure that a variableoccurrence of power as well as a variable demand can be compensated for.After the energy store, the medium 3 drives an engine 7, preferably asteam turbine, which transfers its energy via the output shaft to anelectric generator 8 and/or to a mechanical consumer 9. The medium isreturned to the liquid state via a condenser 10 and is re-pressurized bya pump 11 and again returned to the circuit.

In an advantageous advanced embodiment, the medium 3 is charged directlyfrom the cooling circuit of the combustion engine 1 via a switch unit12. In normal operation, the medium circulates in a closed circuit.Under certain operating conditions, such as, e.g., in a cold start, thetwo circuits can be interconnected so that the warmer one preheats theother one.

In another embodiment according to the invention, the engine 7 is apiston engine, either a reciprocating piston engine or a rotating pistonengine, or a gas turbine.

In a further embodiment according to the invention, the heat transfermeans 2 can be a heat pump for increasing the temperature level of themedium 3 beyond that of the waste heat from the thermal process of thecombustion engine 1.

According to the invention and according to FIG. 2, the heat of theexhaust gas, which must be cooled for the EGR, is used such that itevaporates the liquid agent flowing through the first heat exchanger(EGR evaporator 1). The energy contained in the vapour can be used foranother energy utilization, before the reliquified vapour again passesthrough the circuit. The energy rendered usable for mechanical purposesis not necessarily discharged via the engine heat exchanger (chiller,radiator). Thus, said heat exchanger can either be designed smaller orcan yield the required cooling capacity for correspondingly higherexhaust gas recirculation rates—involving a corresponding benefit interms of a decrease in the emissions of nitrogen oxide.

Since engines operating on the expansion of steam exhibit a narrowoptimal operating range, in a special embodiment, the mass flow and thepressure applied on the engine are limited by a waste-gate. In saidembodiment, the surplus portion of the vapour generated in the processis added to the combustion air. This is a process known per se whichalso serves for the purpose of reducing the amount of nitrogen oxide. Adirect coupling of those measures is advantageous, since the operatingranges which exhibit high recoverable thermal energy flows (full load)are also those ranges in which the discharge of nitrogen oxide emissionsreaches its peak. However, in this operating mode, the vapour is used upso that it becomes necessary to refill the system (FIG. 3).

The vapour can be mixed with a vapour generated otherwise in order toincrease, in this manner, the volume rather than the temperature (FIG.4, 5, 6). An increased vapour volume can be used for efficiency purposesin analogy to a vapour compressed by pressure. The vapour generatedotherwise can originate both from energy sources of a heat engine andfrom a fuel cell. The coupling of all heat sources is also providedaccording to the invention (FIG. 6).

However, due to the higher temperature of the exhaust gas in the exhaustgas recirculation circuit, the energy from the EGR can also be used forsuperheating a vapour already generated otherwise (FIG. 7), which thencan drive, e.g., an engine connected to a generator and/or mechanicalconsumers via a drive shaft.

The individual evaporators according to FIGS. 4, 5 and 6 are operated infeedback with the evaporator output so that equal pressure conditionsprevail in the evaporator circuits and it becomes possible to mix thevapour generated in the evaporator connected in parallel. This isachieved by means of output-controlled pumps P1, P2 and P3.

However, according to FIG. 7, two evaporators connected in series areprovided.

1. A process for the recovery of energy from the waste heat of acombustion engine comprising an exhaust gas recirculation, in particularof a mobile combustion engine, characterized in that at least a portionof the waste heat of the recycled exhaust gas evaporates a liquid and/orheats a vapour and/or a gas, increasing the pressure thereof, and thispressure is transformed into mechanical energy in an engine.
 2. Aprocess for the recovery of energy from the waste heat of a combustionengine, in particular of a mobile combustion engine, and of a fuel cell,characterized in that at least a portion of the waste heat of theexhaust gas of the combustion engine, in particular of a recycledexhaust gas, and at least a portion of the waste heat of the fuel cellevaporate a liquid and/or heat a vapour and/or a gas, increasing thepressure thereof, and this pressure is transformed into mechanicalenergy in an engine.
 3. A process according to claim 1, characterized inthat the vapour or gas is evaporated or heated, respectively, in two ormore stages.
 4. A process according to claim 1, characterized in thatthe conversion into mechanical energy is carried out via a steamturbine.
 5. A process according to claim 1, characterized in that theconversion into mechanical energy is carried out via a gas turbine.
 6. Aprocess according claim 1, characterized in that the conversion intomechanical energy is carried out via a piston engine.
 7. A processaccording to claim 1, characterized in that at least one heat transferstage is a heat pump.
 8. A process according to claim 1 characterized inthat, in front of the engine, energy is stored in an energy store.
 9. Aprocess according to claim 1, characterized in that the mechanicalenergy is used as an auxiliary energy for the combustion engine and/orfor the fuel cell and/or as an auxiliary energy in means of transportsuch as vehicles, preferably for driving a coolant pump and/or ahydraulic unit and/or a compressor for an air-conditioning system.
 10. Aprocess according to claim 1, characterized in that the mechanicalenergy is transformed into electrical energy.
 11. A process according toclaim 10, characterized in that the electrical energy is used as anauxiliary energy for the combustion engine and/or for the fuel celland/or as an auxiliary energy in vehicles, preferably for driving acoolant pump and/or a hydraulic unit and/or a compressor for anair-conditioning system.
 12. A process according to claim 1,characterized by an exhaust gas turbine of the combustion engine,wherein exhaust gas to be recycled is branched off before the exhaustgas is introduced into the exhaust gas turbine.
 13. A process accordingto claim 12, characterized in that at least a portion of the waste heatof the exhaust gas expanded in the exhaust gas turbine evaporates aliquid and/or heats a vapour and/or a gas, increasing the pressurethereof, and this pressure is transformed into mechanical energy in anengine.
 14. A process according to claim 12, characterized in that theliquid or the vapour or the gas, respectively, heated by the expandedexhaust gas, is heated further, in particular superheated, by theexhaust gas to be recycled.
 15. A process according to claim 12,characterized in that the liquid or the vapour or the gas, respectively,heated by the expanded exhaust gas, is mixed with the liquid or thevapour or the gas, respectively, heated by the recycled exhaust gas. 16.A process according to claim 2, characterized in that a liquid or avapour or a gas, respectively, heated by the waste heat of the fuel cellis mixed with a liquid or a vapour or a gas, respectively, heated by theexhaust gas of the combustion engine.
 17. A device for the recovery ofenergy from the waste heat of a combustion engine comprising an exhaustgas recirculation, in particular of a mobile combustion engine,characterized by the combination of the following features: at least oneheat transfer means for transferring the thermal energy of the recycledexhaust gas to a heat carrier medium, a device for increasing thepressure of the heat carrier medium, an engine, preferably a steamturbine, which transforms the energy stored in the heat carrier mediuminto mechanical energy.
 18. A device for the recovery of energy from thewaste heat of a combustion engine, in particular of a mobile combustionengine, and of a fuel cell, characterized by the combination of thefollowing features: at least one heat transfer means for transferringthe thermal energy of the exhaust gas of the combustion engine to a heatcarrier medium, at least one heat transfer means for transferring thethermal energy of the fuel cell to a heat carrier medium, a device forincreasing the pressure of the heat carrier medium, an engine,preferably a steam turbine, which transforms the energy stored in theheat carrier medium into mechanical energy.
 19. A device according toclaim 17, characterized by two or more heat transfer means whichgradually heat the heat carrier medium.
 20. A device according to claim17, characterized in that the heat carrier medium is identical with thecooling medium for the combustion engine and/or the fuel cell.
 21. Adevice according to claim 17, characterized in that at least one heattransfer means is a heat pump.
 22. A device according to claim 17,characterized in that an energy store is arranged in front of theengine.
 23. A device according to claim 17, characterized in that theengine is coupled with at least one drive for at least one auxiliarypower unit for the thermal or chemical process, preferably cooling orlubricant pumps.
 24. A device according to claim 17, characterized inthat the engine is coupled with at least one drive for at least oneauxiliary power unit for a vehicle, preferably with the drive of ahydraulic unit and/or a compressor for an air-conditioning system
 25. Adevice according to claim 17, comprising an electric power generator,preferably a generator, which is drivable by the engine and transformsat least a portion of the mechanical energy into electrical energy. 26.A device according to claim 25, characterized in that the electricalenergy is provided for the operation of auxiliary power units for thecombustion engine and/or the fuel cell, preferably of cooling orlubricant pumps.
 27. A device according to claim 25, characterized inthat the electrical energy is provided for the operation of auxiliarypower units for a vehicle, preferably of a hydraulic unit and/or acompressor for an air-conditioning system.
 28. A device according toclaim 17, characterized by an exhaust gas turbine of the combustionengine, wherein an exhaust-gas branch duct for exhaust gas to berecycled, which branches off—in the flow direction of the exhaustgas—from the exhaust gas duct in front of the exhaust gas turbine, runsinto a heat transfer means.
 29. A device according to claim 28,characterized in that a heat transfer means for exhaust gas expanded inthe exhaust gas turbine is arranged downstream of the exhaust gasturbine.
 30. A device according to claim 28, characterized in that aheat carrier medium duct runs from the heat transfer means arrangeddownstream of the exhaust gas turbine to the heat transfer means for theexhaust gas to be recycled.
 31. A device according to claim 28,characterized in that a mixing device for the heat carrier medium heatedby the exhaust gas to be recycled and the heat carrier medium heated bythe remaining exhaust gas is provided.
 32. A device according to claim28, characterized in that a heat carrier medium heated by the fuel cellcan be supplied via a duct to a mixing device for mixing with a heatcarrier medium heated by the exhaust gas of the combustion engine.